The interaction of calmodulin with Ca 2 ion and several of its intracellular target proteins including cyclic nucleotide phosphodiesterase, myosin light-chain kinase, phosphorylase, calcineurin and troponin I has been studied in order to understand the unique mechanism of regulation of Ca 2 ion-dependent processes by this protein. Interaction of calmodulin with its target proteins does not always require Ca 2 ion but the subsequent activation of the target enzyme is always dependent on Ca 2 ion. Some of these enzymes appear to contain a calmodulin-binding domain or subunit which serves an inhibitory function in the absence of calmodulin. Since we were unable to isolate a ternary complex of 2 enzyme molecules attached to a single calmodulin it appears that the same interaction site in calmodulin may be recognized by different proteins or that binding at one site prevents binding at other sites. Regulation of calmodulin-dependent reactions by cAMP-dependent phosphorylation and by other intracellular Ca 2 ion binding proteins indicates that the two second messengers Ca 2 ion and cyclic nucleotides are tightly coupled.